1. Field of the Invention
Exemplary embodiments of the present invention relate to a technology of transmitting a long training field in a radio communication system, and more particularly, to a method and an apparatus of efficiently transmitting a long training field (LTF) in accordance with the number of streams allocated to each user in a multi-user multiple input multiple output (MIMO) based radio communication system.
2. Description of Related Art
Various transmission technologies capable of providing a high transmission rate have been currently researched in a radio communication field. IEEE 802.11 test group n (TGn) is organized in order for a wireless LAN system to provide a high transmission rate. IEEE 802.11 TGn standardizes a wireless LAN system having a maximum transmission rate of 600 Mbps to which a multiple input multiple output (MIMO) technology is applied.
FIG. 1 illustrates two protocol data units that are defined in the IEEE 802.11 TGn. An (a) in FIG. 1 illustrates a physical layer convergence procedure (PLCP) protocol data unit (PPDU) in a high throughput (HT) mixed format in which a legacy station and an HT station are mixed, and (b) in FIG. 1 illustrates the PLCP protocol data unit (PPDU) in an HT Greenfield format in which only the HT station is present.
Referring to FIG. 1, the PPDU in the format in which the legacy station and the HT station are mixed includes training symbol fields (L-STF and L-LTF) and a signal field (L-SIG) for the legacy station and a signal field (HT-SIG), training symbol fields (HT-STF and HT-LTF), and a data field (Data) for the HT station.
Further, referring to FIG. 1, the PPDU under the Greenfield in which only the HT station is present includes a short training symbol (HT-GE-STF) and a long training symbol (HT-LTF1), an HT signal field (HT-SIG), HT long training symbols (HT-FLT) for the HT Greenfield, a data field (Data) for the HT Greenfield.
Further, in the IEEE 802.11 TGn, a single station (STA) is defined to transmit a maximum of four streams. Each stream transmits the LTF symbols using a long training field (LTF) mapping code depending on Equation 1. When a spatial stream is smaller than four, a sub-matrix of the LTF mapping code depending on Equation 1 is used.

For example, when a transmitting antenna and a receiving antenna, respectively, are two and a transmitting stream is two, if it is assumed that an MIMO channel to be estimated is expressed by Equation 2, the received signal to which the LFT mapping code is applied is expressed by the following Equation 3.
                                              ⁢                  H          =                      [                                                                                h                    00                                                                                        h                    01                                                                                                                    h                    10                                                                                        h                    11                                                                        ]                                              [                  Equation          ⁢                                          ⁢          2                ]                                y        =                                                            [                                                                                                    h                        00                                                                                                            h                        01                                                                                                                                                h                        10                                                                                                            h                        11                                                                                            ]                            ⁢                                                p                                      2                                                  ⁡                                  [                                                                                    1                                                                                              -                          1                                                                                                                                    1                                                                    1                                                                              ]                                                      +                          [                                                                                          n                      00                                                                                                  n                      01                                                                                                                                  n                      10                                                                                                  n                      11                                                                                  ]                                =                                                    1                                  2                                            ⁡                              [                                                                                                                              h                          00                                                +                                                  h                          01                                                                                                                                                              -                                                      h                            00                                                                          +                                                  h                          01                                                                                                                                                                                                  h                          10                                                +                                                  h                          11                                                                                                                                                              -                                                      h                            10                                                                          +                                                  h                          11                                                                                                                    ]                                      +                          [                                                                                          n                      00                                                                                                  n                      01                                                                                                                                  n                      10                                                                                                  n                      11                                                                                  ]                                                          [                  Equation          ⁢                                          ⁢          3                ]            
In this case, a subcarrier index is omitted in the received signals for convenience and thus, the LTF field p also has different values in accordance with the subcarrier. Therefore, the estimated channel is expressed by the following Equation 4. In the following Equation 4, n represents noise.
                              H          ^                =                                                                              p                                      2                                                  ⁡                                  [                                                                                                                                          h                            00                                                    +                                                      h                            01                                                                                                                                                                            -                                                          h                              00                                                                                +                                                      h                            01                                                                                                                                                                                                                    h                            10                                                    +                                                      h                            11                                                                                                                                                                            -                                                          h                              10                                                                                +                                                      h                            11                                                                                                                                ]                                            ⁢                                                                    p                    *                                                        2                                                  ⁡                                  [                                                                                    1                                                                    1                                                                                                                                      -                          1                                                                                            1                                                                              ]                                                      +                                          [                                                                                                    n                        00                                                                                                            n                        01                                                                                                                                                n                        10                                                                                                            n                        11                                                                                            ]                            ⁢                                                                    p                    *                                                        2                                                  ⁡                                  [                                                                                    1                                                                    1                                                                                                                                      -                          1                                                                                            1                                                                              ]                                                              =                                                                                        p                                                  2                            ⁡                              [                                                                                                    h                        00                                                                                                            h                        01                                                                                                                                                h                        10                                                                                                            h                        11                                                                                            ]                                      +                                                            p                  *                                                  2                                            ⁡                              [                                                                                                                              n                          00                                                -                                                  n                          01                                                                                                                                                              n                          00                                                +                                                  n                          01                                                                                                                                                                                                  n                          10                                                -                                                  n                          11                                                                                                                                                              n                          10                                                +                                                  n                          11                                                                                                                    ]                                                                        [                  Equation          ⁢                                          ⁢          4                ]            
An LTF mapping code PHTLTF for the HT station has unitary characteristic. Therefore, the number (NDLTF) of LTFs depending on the number (NSTS) of spatial streams is as shown in the following Table 1.
TABLE 1NSTSNDLTF11223444
In Table 1, the STS represent a spatial time stream. It is assumed in the exemplary embodiment of the present invention that the number of spatial streams is equal to the number of spatial time streams. Further, the DLTF represents a data long training field. It is assumed in the exemplary embodiment of the present invention that the number of data long training fields is equal to the number of long training fields.
Recently, IEEE 802.11 VHT Study Group (SG) is discussing a wireless LAN system capable of providing a transmission rate of a maximum of 1 Gbps in a MAC service access point (SAP) and configures a Task Group of IEEE 802.11 TGac/TGad. In order to maintain frequency efficiency while meeting the high transmission rate, an access point (AP) and a station (STA) need to support streams more than four streams supported in the Task Group n (TGn), such that a large number of antennas are required.
In an aspect of the station (STA), it is difficult to support a large number of antennas when considering complexity or power consumption of the station. Therefore, a multi-user MIMO technology for allowing the access point AP to simultaneously transmit data to the plurality of stations (STAs) has been considered. When the same type as the Task Group n (TGn) is expanded to a multi-user MIMO type so as to transmit at least four streams, a need exists for a new format capable of support at least four streams. A very high throughput (VHT) wireless LAN system uses the multi-user MIMO technology to consider the PLCP protocol data unit (PPDU) format as illustrated in FIG. 2.
FIG. 2 illustrates the PLCP protocol data unit (PPDU) format in a VHT mixed format in which the legacy station, the HT station, and the VHT stations are mixed.
Referring to FIG. 2, the PLCP protocol data unit (PPDU) for the VHT includes the training symbol fields (L-STF and L-LTF) for the legacy station, the signal field (L-SIG) for the legacy station, a signal field (VHT-SIG-C) for the VHT station that can be decoded by all the stations, the training symbol fields (VHT-STF and VHT-LFT) for the VHT station, the signal field for the VHT station (VHT-SIG-D that can be decoded by only the VHT station, and the data field (Data).
If it is assumed that four stations (STAs) each transmit two streams, 8 LTF symbols are required so as to perform the channel estimation without interference between the stations (STAs). When the total number of transmission streams is 8, the LTF mapping code depending on Equation 5 may be created by expanding the mapping code of the Task Group n (TGn). When the number of spatial streams is smaller than 8, the sub-matrix of the LTF mapping code is used.
                              P          VHTLTF                =                  [                                                    1                                                              -                  1                                                            1                                            1                                            1                                                              -                  1                                                            1                                            1                                                                    1                                            1                                                              -                  1                                                            1                                            1                                            1                                                              -                  1                                                            1                                                                    1                                            1                                            1                                                              -                  1                                                            1                                            1                                            1                                                              -                  1                                                                                                      -                  1                                                            1                                            1                                            1                                                              -                  1                                                            1                                            1                                            1                                                                    1                                                              -                  1                                                            1                                            1                                                              -                  1                                                            1                                                              -                  1                                                                              -                  1                                                                                    1                                            1                                                              -                  1                                                            1                                                              -                  1                                                                              -                  1                                                            1                                                              -                  1                                                                                    1                                            1                                            1                                                              -                  1                                                                              -                  1                                                                              -                  1                                                                              -                  1                                                            1                                                                                      -                  1                                                            1                                            1                                            1                                            1                                                              -                  1                                                                              -                  1                                                                              -                  1                                                              ]                                    [                  Equation          ⁢                                          ⁢          5                ]            
When considering that the LTF mapping code PVHTLTF for the VHT station has the unitary characteristic, the relationship between the number of streams and the number of LFTs in the LTF mapping code is as shown in Table 2. Table 2 shows the relationship between the number (NSTS) of spatial streams and the number (NDLTF) of LTFs when the LTF mapping code of the TGn for the VHT is expanded.
TABLE 2NSTSNDLTF1122344458687888
As can be appreciated from Table 2, 8 LTFs are required even when 5, 6, and 7 streams are transmitted. Therefore, using the expanded mapping code of the Task Group n (TGn) increases the overhead of the LTF.
Meanwhile, as another mapping code, a type such as discrete Fourier transform (DFT) may be applied. In this case, the relationship between the total number of transmission streams and the number of LTFs is as shown the following Table 3. Table 3 shows the relationship between the number of spatial streams and the number of LTFs when an orthogonal mapping code such as the DFT for the VHT is applied.
TABLE 3NSTSNDLTF1122334455667788
As can be appreciated from Table 3, 8 LTFs are required to transmit eight streams. Increasing the number of LTFs increases a weight occupied by the LTF in the PLCP protocol data unit (PPDU) format, such that there is a problem in that the transmission efficiency is degraded.
For example, when considering the MU-MIMO environment in which two stations (STA) each transmit four streams of a packet of 1500 bytes simultaneously, the access point (AP) may transmit a total of eight streams. Comparison of the transmission efficiency in this case and the transmission efficiency of the case of the TGn transmitting four streams is as shown in Table 4.
TABLE 4The number ofTransmission EfficiencyThe number ofData symbols (40(%) The number of dataLTFs (STF,MHz bandwidth,symbols/Total number ofLTF, SIG)600 Mbps/STA)symbols of PPDU formatTGn4633%VHT8626%
That is, when the LTF mapping type as shown in [Table 2] and [Table 3] is applied, it can be appreciated that the data transmission efficiency may greatly degraded in the VHT wireless LAN system. When the number of user stations (STAs) performing simultaneous transmission in the VHT wireless LAN system is more increased, the number of LTFs is more increased. In this case, the transmission efficiency may be more reduced than that of the TGn wireless LAN system.